Dexmethylphenidate, also known as d-threo-methylphenidate, (R,R)-methylphenidate or (R,R)-α-phenyl-2-piperidineacetic acid methyl ester, having the formula:
is CNS (central nervous system) stimulant that is chemically and pharmacologically similar to the amphetamines. Dexmethylphenidate's CNS actions is milder than those of the amphetamines and have more noticeable effects on mental activities than on motor activities.
It has been reported by Sporzny (1961) that among racemic mixtures of threo and erythro diastereomers of methylphenidate, only threo-isomer displays stimulant properties. Dexmethylphenidate hydrochloride (i.e. the d-threo enantiomer of methylphenidate hydrochloride) has been reported to be 5 to 38 times more active than the corresponding (S,S)-methylphenidate hydrochloride (Prashad 2000).
A commercially available drug is sold under the name Focalin™ (Novartis) and it consists of dexmethylphenidate in the form of the hydrochloride salt. This product is orally administered and clinically used in the treatment of narcolepsy and as adjunctive treatment in children with attention deficit disorder (ADD) and attention-deficit hyperactivity disorder (ADHD).
A synthesis of dexmethylphenidate hydrochloride was firstly described in U.S. Pat. No. 2,838,519 and include resolution of erythro-α-phenyl-2-piperidineacetamide to obtain enantiopure (2R,2′S)-α-phenyl-2-piperidineacetamide, which was subjected to epimerization, hydrolysis, and esterification as shown in Scheme 1:

Related example of preparation of dexmethylphenidate from erythro-α-phenyl-2-piperidineacetamide was described in U.S. Pat. No. 5,936,091.
Preparation of dexmethylphenidate through optical resolution of threo-α-phenyl-2-piperidineacetamide was described in U.S. Pat. No. 5,965,734, as shown in Scheme 2:

Synthetic methods for the preparation of racemic mixture of threo- and erythro-α-phenyl-2-piperidineacetamides as raw materials for the preparation of dexmethylphenidate were described by Panizzon (1944) and Patric (1982) and in U.S. Pat. Nos. 2,507,631, 2,838,519, 2,957,880 and 5,936,091, and in WO 01/27070. These methods include using sodium amide as base in the nucleophilic substitution of chlorine in 2-chloropyridine with phenylacetonitrile followed by hydrolysis of the formed nitrile and reduction of a pyridine ring to a piperidine one by hydrogenation on PtO2 catalyst, as shown in Scheme 3:

Alternatively, 2-bromopyridine was used instead of 2-chloropyridine by Deutsch (1996).
In some other methods threo-methylphenidate was used as the raw material for the preparation of dexmethylphenidate. Threo-methylphenidate may be prepared by a several routes, inter alia by the following two processes:
i) by esterification of threo-ritalinic acid which may be prepared from erythro-enriched and threo-α-phenyl-2-piperidineacetamides as shown in Scheme 4:

ii) by cyclization of easily available 1-(phenylglyoxylyl)piperidine arenesulfonylhydrazone to (R*,R*)-enriched 7-phenyl-1-azabicyclo[4.2.0]octan-8-one and further converting the β-lactam to threo-methylphenidate hydrochloride, as described by Axten (1998), Corey (1965) and Earle (1969) and in WO 99/36403 and shown in Scheme 5:

The resolution of threo-methylphenidate to afford dexmethylphenidate was first reported by Patric (1987) which used (R)-(−)-binaphthyl-2,2′-diyl hydrogen phosphate as the resolving agent. Several new resolutions of threo-methylphenidate have been reported recently by Prashad (1999) and in U.S. Pat. Nos. 6,100,401, 6,121,453, 6,162,919 and 6,242,464 as described in Scheme 6:
wherein the chiral acid is one of the following: (R)-(−)-binaphthyl-2,2′-diyl hydrogen phosphate, (−)-menthoxyacetic acid, ditoluoyl-D-tartaric acid or dibenzoyl-D-tartaric acid.
Resolution of threo-methylphenidate may be also achieved by enzymatic hydrolysis methods as proposed by Prashad (1998) and in WO 98/25902. Such resolution is described in Scheme 7:

Resolution of threo-ritalinic acid hydrochloride with (S)-1-phenylethylamine give complex salt (R,R)-enriched threo-ritalinic acid.HCl.(S)-1-phenylethylamine with 77% ee optical purity of ritalinic acid (U.S. Ser. No. 2002/0019535), Scheme 8:
